In a typical CW radar cross-section measurement range, the target signal competes with the background signal, from which it cannot be distinguished. Depending on the minimum radar signal to be measured, it is therefore necessary to make the target background of low reflectivity. This is achieved either by radar absorbent material in an anechoic chamber or by free space. Radar absorbent material of low reflectivity over a wide range of frequencies and reflection angles is difficult to make. Free space is often inconvenient or impractical for a target background.
Likewise, the target signal cannot be distinguished from a leakage signal travelling directly between transmitter and receiver. In a CW radar which uses the same antenna for transmission and reception, separating the received signal from the leaked transmit signal generally involves careful cancellation of the leakage signal with another signal of equal amplitude and opposite phase. Cancellation between the two signals is affected by frequency drift of the transmit source and by thermal change of path lengths inside the radar, and therefore, it is difficult to maintain perfect cancellation over extended time periods. If separate antennas are used for transmission and reception in backscatter and bistatic systems, there is usually coupling between antenna sidelobes. The signal received on this path also requires cancellation.
This invention will allow one to measure a CW radar signal scattered from a stationary target in the presence of large background clutter and leakage signals without a need to reduce these signals to a level small compared with the target signal. The invention has some similarity to a single delay canceller in a moving target indication (MTI) radar, as described in the Radar Handbook, Chapter 17, McGraw Hill, 1970, M. I. Skolnik, Editor. The single delay MTI canceller subtracts the echoes of a first pulse received by a stationary pulse radar from the echoes of a second pulse received one over the pulse repetition frequency later. The echoes of the first pulse are delayed by exactly the inverse of the pulse repetition frequency in a radar-internal delay line before subtraction. Clutter signals, such as received from background terrain, are identical in both pulses and subtract out. Echoes from moving targets except for those that have moved an integral numbers of half radar wavelengths toward or away from the radar between two pulses do not subtract out. Substantial sub-clutter visibility of moving targets is thus achieved by an MTI radar.